The present invention generally relates to thermocouples and to the method of making thermocouples. More specifically, the invention is related to thermocouples made by thick film technology wherein metal pastes containing frit are applied by known printing or coating techniques to a suitable surface, subsequently fired and assembled as a thermocouple.
Thermocouples have been in widespread use for many years. The small electrical voltage generated at the junction of two dissimilar metals changes in proportion to the temperature of the junction (relative to a reference temperature). With the standardized thermocouples in common use a reasonably linear response of the voltage produced to the temperature is obtained.
Measurement of high temperatures with thermocouples is possible; however, due to the deterioration of the metals at high temperatures, difficulties in maintaining a stable voltage have been experienced. In U.S. Pat. No. 3,066,177, the inferior performance at temperatures above 1100.degree. C typical of Chromel-Alumel.RTM. thermocouples is discussed, and a replacement for such thermocouples which employs precious metals and exhibits long term stability at high temperatures was disclosed and claimed. While Chromel-Alumel.RTM. thermocouples have a positive leg of 90 percent nickel and 10 percent chromium and a negative leg of 94 percent nickel, 3 percent manganese, 2 percent aluminum and 1 percent silicon, the improved thermocouples, which are available commercially under the name Platinel.RTM., have a positive leg of 50-85 percent palladium, 10-35 percent platinum, and 2-15 percent gold and a negative leg of 60-70 percent gold and 30-40 percent palladium.
In particular, two compositions are presently commercially produced: Platinel.RTM.I, having a positive leg of the nominal composition 83 percent palladium, 14 percent platinum, 3 percent gold, all by weight, and a negative leg having a nominal composition of 65 percent gold and 35 percent palladium by weight; and Platinel.RTM.II, having a positive leg of the nominal composition 55 percent palladium, 31 percent platinum, and 14 percent gold by weight and a negative leg having a nominal composition of 65 percent gold and 35 percent palladium by weight. These compositions match the standard Chromel-Alumel.RTM. thermocouple performance, (which itself meets those for Type K thermocouples set by the Instrument Society of America) and they can be used with the same electronic measuring equipment as the Chromel-Alumel.RTM. thermocouples.
Typically thermocouples are constructed with solid wire, as are the Chromel-Alumel.RTM. and Platinel.RTM. thermocouples mentioned above. In U.S. Pat. No. 3,099,575, thermocouples are described which are made by thick film technology as used in thick film hybrid micro-circuits. The thermocouple legs are made of components selected from the group consisting of platinum, palladium, iridium, rhodium, and gold. No frit is utilized in the manufacture of such thermocouple films. However, such thermocouples have unsatisfactory adhesion to the surface on which they are placed and their electrical performance is inconsistent.
Glass frits of various sorts are widely used in the electronic thick film industry as binders and their relative amounts may vary widely. In the manufacture of conductors, the metal conductivity is achieved essentially by using sufficient metal and is not significantly affected by the frit. However, the composition of the frit can be important in resistor applications. For example, U.S. Pat. No. 3,207,706 discusses the effect of frits on the Temperature Coefficient of Resistance (TCR) and resistance stability (drift). Since electronic applications typically operate at low temperatures, such as military circuit specifications of -55.degree. C to 155.degree. C, the performance of the frits at high temperatures of operation is of little concern.
It is well known in the thermocouple art that the composition of thermocouple wires must be carefully controlled in order to assure consistent electrical performance. Therefore, it would be expected by those skilled in the thermocouple art that addition of extraneous materials such as a frit to the thick film paste would have deleterious effects on the electrical performance of the thermocouple.
Thick films including a metal component and a frit have been successfully used in the manufacture of resistance thermometers, of which U.S. Pat. No. 3,781,749 is an example. However, in contrast to the thermoelectric effect created in thermocouples by use of two dissimilar elements, the variation of electrical resistance of a single element with temperature is used to measure temperature in a resistance thermometer. A survey in Instrument Practice, May 1966, discusses some of such applications, which in general involve measurement of transient conditions where the rapid response of a thick film is of particular value.
It would be desirable to form thick film thermocouples which could adequately withstand high temperature exposure and retain the electrical stability and physical durability typical of the precious metal thermocouples presently used. Such thick film thermocouples would be less expensive and could directly replace solid wire thermocouples in many applications. For other applications, thick film thermocouples would be preferred to obtain rapid response to temperature changes and other inherent advantages of directly applying the thermocouple legs to the surface being measured. Preferably, they would be formulated to achieve an EMF response matching that of the standard solid wire thermocouples. However, non-standard thermocouples could be used if measuring equipment were designed especially to suit the voltage they produce in response to changes in temperature.
In accordance with the present invention, there is disclosed a thick film thermocouple using a frit and exhibiting substantially improved mechanical properties while at the same time having essentially no effect on the electrical response. Such thick film thermocouples are disclosed below in greater detail.